More precisely, it relates to the control of changes of gear ratio of an automatic transmission programmed with distinct shifting rules or variograms, applied according to the operating conditions of the vehicle.
According to a traditional system, illustrated in particular by French Patent 2545567, the decisions to change year ratios of a transmission with discrete gear ratios are made when a point representative of the operation of the transmission in a coordinate plane defined by speed of travel of the vehicle and engine load (accelerator pedal position, engine torque power etc.) intersects an upshift curve or a downshift curve. In this representation, the set of upshift and downshift curves defines a shifting rule that controls, for the transmission, the set of situations encountered by the vehicle, its power train and its operator.
In the case of infinitely or continuously variable transmissions, the variations of gear ratios are controlled by a map or variogram comprising an analog grouping of a family of shift curves covering the entire range of transmission ratios.
The shifting rules and the variograms are generally established in such a way that fuel consumption is as low as possible, subject to acoustic comfort and satisfactory acceleration. Under these conditions, upshifts at low and medium load are most often effected at low engine speed.
On these basic principles there were developed what are known as “auto-adaptive” transmissions, which can comprise as illustrated by French Patent 2741931, several shifting rules, adapted to diverse traffic conditions (city, uphill, downhill, etc.) to the driving style of the operator, etc., in order to improve driving pleasure under all circumstances. Transmissions programmed with auto-adaptive rules are capable of switching between different shifting runes or variograms (when the gear ratio is continuously variable).
A switch of shifting rules or of variograms can be felt immediately by a change of gear ratio, which may surprise the operator. In fact, such a change of gear ratio results primarily from a response of the transmission to the evolution of its own operation and not to a modification of the operator's intent or behavior and/or of the vehicle. Such a situation may be encountered under similar conditions in the case of transmissions with discrete transmission ratios and in that of continuous transmissions.